Development of integrated photoplethysmographic recording circuit for trans-nail pulse-wave monitoring system

Health monitoring and self-management have become increasingly more important because of health awareness improvement, the aging of population, and other reasons. In general, pulse waves are among the most useful physiological signals that can be used to calculate several parameters such as heart rate and blood pressure for health monitoring and self-management. To realize an automatic and real-time pulse-wave monitoring system that can be used in daily life, we have proposed a trans-nail pulse-wave monitoring system that was placed on the fingernail to detect photoplethysmographic (PPG) signals as pulse waves. In this study, we designed a PPG recording circuit that was composed of a 600 x 600 mu m(2) photodiode (PD), an LED driver with pulse wave modulation (PWM) and a low-frequency ring oscillator (RING), and a PPG signal readout circuit. The proposed circuit had a very small area of 2.2 x 1.1 mm(2) designed with 0.18 mu m CMOS technology. The proposed circuit was used to detect pulse waves on the human fingernail in both the reflection and transmission modes. Electrical characteristics of the prototype system were evaluated precisely and PPG waveforms were obtained successfully. (C) 2018 The Japan Society of Applied Physics.